Liquid crystals consist of anisotropic molecules. The average direction of the long molecular axis is called the director, d. The director orientation is determined by the anchoring of the liquid crystal on rigid substrates and is characterized by the direction of the axis of easy director orientation, e, and anchoring energy W.
Reorientation of the director caused by the application of an external electric field is the basis of operation of liquid crystal displays. The basic unit of liquid crystal devices is a liquid crystal cell, which includes two rigid substrates with a liquid crystal sandwiched between. To obtain uniform brightness and high contrast ratio it is desired to produce a uniform alignment of liquid crystals in the cell.
To produce uniform planar orientation of liquid crystals, several techniques involving different polymer materials are generally used.
One technique is the rubbing method. Polymer layers are deposited on the substrate and rubbed unidirectionally. The director, d, is usually aligned parallel to the direction of rubbing in the plane of the substrate. A pretilt angle, φ, between the substrate and the director in the plane perpendicular to the substrate may be produced by this method.
The rubbing method produces stable planar alignment with strong anchoring. However, this technique has some drawbacks. In particular, dust and static electricity generated during the rubbing can cause defects in liquid crystal displays. Moreover, it is difficult to orient selected regions of the liquid crystal surface locally so that each region has a different orientation. It is difficult to obtain multi-domain alignment.
Another technique is the photo aligning method. Photosensitive polymer layers are deposited on the substrate and are irradiated by polarized UV light. Such layers possesses a light induced anisotropy axis that produces high quality planar alignment of the liquid crystal molecules in a preferred axial direction perpendicular or parallel to the polarization vector of the UV light beam, E. Tilted alignment can be obtained by oblique irradiation of the polymer layer.
The photo aligning method produces stable planar and tilted alignment of most commercial nematic liquid crystals. In contrast to rubbing, no electrostatic charges or dust are produced on the aligning surface. Also, the direction of the easy axis and the anchoring energy can be locally varied by changing the direction of light polarization and the time of UV exposure.
An example of the photo aligning method can be found in U.S. Pat. No. 5,389,698 to V. Chigrinov et al, which uses a photopolymer polyvinyl-cinnamate aligning layer irradiated with plane-polarized light. Another example of the photo aligning method can be found in U.S. Pat. No. 5,807,498 to Gibbons et al, which uses polyimides with di-aryl ketones and di-aryl ketones alignment layers.
Both of the above methods use special polymer materials to produce the alignment of the liquid crystals.
Another method uses light irradiation of a liquid crystal cell filled with dye-doped liquid crystals. This method can produce planar alignment of liquid crystals (Jap.Journ.Appl.Phys. v.34 (1995) 566). The mechanism of the alignment is postulated to be a result of absorption of the light by the dye molecules followed by their anisotropic adsorption onto the substrate. This method, however, requires the use of dye-doped liquid crystals to form an alignment layer.
U.S. Pat. No. 5,032,009 to Gibbons et al. discloses exposing anisotropically absorbing molecules that are on a substrate, disposed in a liquid crystal medium, and the liquid crystals themselves to linearly polarized light. However, non-mesogenic molecules, such as a polyimide, are coated onto the substrate and exposed to linearly polarized light to produce alignment.
What is needed in the art is an alignment layer that can be formed from light irradiated liquid crystals.
It is therefore an object of the invention to provide a method of forming an alignment layer made from a liquid crystal film that is irradiated with light.
It is another object of the invention to provide a method of forming a liquid crystal cell that has at least one alignment layer made from a liquid crystal film that is irradiated with light.